Accident Overview
On October 24, 2024, at approximately 1535 Hawaii-Aleutian standard time, a Cessna 150 (registration N10495) was involved in an accident near Lanai, Hawaii. The aircraft was operating as a Title 14 Code of Federal Regulations Part 91 personal flight, carrying a pilot and one passenger, both of whom were uninjured. The airplane sustained substantial damage, primarily to the left wing.
Sequence of Events
The pilot reported that after climbing to about 800 ft above ground level, the engine abruptly ceased producing power. The pilot applied full carburetor heat, which temporarily restored partial power. Upon reducing carburetor heat, power was again lost. Reapplication of full carburetor heat yielded some power, but insufficient to maintain altitude. The pilot subsequently performed an emergency landing on a dirt road approximately three miles north of the airfield. The left wing incurred substantial damage during the landing.
Post-Accident Examination
Fuel (automotive gasoline, or Mogas) drained from the carburetor bowl showed no particulates or water. Continuity was verified for the throttle, mixture, and carburetor heat controls. Spark plug electrodes exhibited normal wear; however, the No. 4 cylinder spark plugs were white in color, while those from other cylinders had a smoky dark gray appearance. All spark plugs produced ignition. Thumb compression was present on all cylinders. A borescope inspection revealed light longitudinal scratches on the No. 4 cylinder from the top of ring travel to the bottom of the piston’s stroke, but all cylinders were deemed serviceable.
Engine operation was tested post-accident: it idled within normal limits. At 1,700 rpm, magneto and carburetor heat functioned normally, and the engine ran smoothly. At 2,400 rpm, the engine ran slightly rough; reducing to 2,000 rpm worsened the roughness. The engine idled smoothly at 700 rpm when warm. The oil filter contained small metal flakes but no glitter. The carburetor (Marvel Schebler MA-3SPA 10-4894-1, with 708.6 hours since overhaul) was removed for further inspection. The carburetor had primarily been operated on unleaded Mogas.
On December 5, 2024, the Federal Aviation Administration (FAA) and Marvel Schebler conducted a flow test and disassembly of the carburetor. The flow test was slightly rich but within normal parameters. Disassembly revealed a small amount of debris on the internal filter, but nothing likely to cause operational issues.
Weather and Carburetor Icing Considerations
At the time of the accident, weather reported at Lanai Airport (about three miles south) included a temperature of 24°C and a dew point of 19°C, yielding a relative humidity of 70%. Review of the carburetor icing probability chart from FAA Special Airworthiness Information Bulletin CE-09-35 indicated that these atmospheric conditions were “conducive to serious icing at glide [idle] power.” The bulletin notes that carburetor icing can occur at temperatures well above freezing, especially with high humidity, due to the cooling effect of fuel vaporization and air expansion. The pilot was operating on Mogas, which, according to Transport Canada publication TP 10737, is generally more volatile than Avgas and can absorb more heat during vaporization, increasing the likelihood of carburetor ice formation at higher ambient temperatures and lower humidity. The publication advises that carburetor heat may need to be applied earlier and for longer duration when using Mogas.
Probable Cause
No probable cause was stated in the source material.